To comprehend El Niño, you
need to understand the interactions between the ocean
and the atmosphere. Warm ocean surface water heats and
adds moisture to the air above it, forming low pressure
systems (Low). Cold surface water cools the air
above it, forming high pressure systems (High).
In a Low, the warm, moist air produces tropical thunderstorms.
In a High, cool, dry air sinks back to the surface and
there is no precipitation. Together, the High and Low
create a circulation pattern in which surface winds
blow toward the Low and upper-level winds blow toward
the High.

Motion within the ocean is also important.
Within the ocean, an invisible boundary called the thermocline
separates warm surface water from cold, deeper water.
A shallow thermocline indicates a small amount of warm
water, and a deep thermocline means there's a lot of
warm water. Because warm water takes up more space than
cold water, average sea level is higher where the thermocline
is deep and lower where the thermocline is shallow.

Meteorologists now recognize a phenomenon related to El Niño called La Niña, in which conditions are nearly opposite those of El Niño. The diagrams and text below compare the features of the ocean and atmosphere that accompany normal, El Niño, and La Niña conditions.

NOAA

Normal Conditions
Under normal conditions, deep warm water in the
western Pacific produces a low pressure region
with heavy storm activity, while the eastern Pacific
is a dry, high pressure area with shallow warm
water. Surface winds blow from east to west, while
upper winds blow from west to east. The thermocline
is deeper, and sea level higher, in the western
Pacific than in the east.

El Niño Conditions
During El Niño, warm surface water appears farther east and is spread over a broader area. Weak Highs form east and west of the Low, and surface and upper level winds are both weaker than normal. The thermocline is deeper and flatter overall, making average sea level of the eastern Pacific higher than normal.

La Niña Conditions
La Niña episodes are characterized by the westward shift of warm water. This produces stronger Highs and Lows, with stronger than normal surface and upper level winds. Warm water is abnormally deep in the western Pacific and abnormally shallow in the eastern Pacific. The slope of the thermocline becomes steeper, and sea level is higher than normal in the west and lower than normal in the east.